1. Field of the Invention
The present invention relates to a capacitive electro-mechanical transducer such as a capacitive micromachined ultrasonic transducer.
2. Description of the Related Art
Recently, much research has been performed on capacitive electro-mechanical transducers that are fabricated by using micromachining processes. A capacitive electro-mechanical transducer usually has cells. A cell includes a lower electrode, a vibrating diaphragm that is supported at a predetermined distance from the lower electrode, and an upper electrode that is disposed on the front surface or back surface of the vibrating diaphragm. Such a capacitive electro-mechanical transducer is used as a Capacitive Micromachined Ultrasonic Transducer (CMUT), for example.
The above-described transducer coverts electrical signals to ultrasonic waves, converts ultrasonic waves to electrical signals, or performs both conversions, by using the lightweight vibrating diaphragm. A transducer of this kind that exhibits excellent broadband characteristics both in liquids and in air can be readily obtained. Since more precise medical diagnosis is possible by using this transducer, attention is being paid to this technique. The principle of operation of this transducer will be described. To transmit ultrasonic waves, a DC voltage on which a very small AC voltage is superposed is applied between the lower electrode and the upper electrode. Thus, the vibrating diaphragm is vibrated and ultrasonic waves are generated. When ultrasonic waves are received, the vibrating diaphragm is deformed by the ultrasonic waves, and changes in the capacitance between the lower electrode and the upper electrode due to the deformation are detected and a signal is thereby obtained. A transducer usually has a plurality of elements, each of which has a plurality of electrically connected cells. In such a configuration, the plurality of elements sometimes vary in sensitivity, and for this reason a method of sensitivity correction has been proposed (Japanese Patent Laid-Open No. 2004-125514). In this method, the output signals produced by ultrasonic-waves-detecting elements are electrically adjusted by a control unit such that the variations in the output signals (variations in the sensitivity) become small.
The sensitivity of the above-described cell or element is inversely proportional to the square of the distance (gap) between the electrodes, for example. Therefore, variations in the distances between the electrodes result in variations in the sensitivity of the transducer. The gap of a capacitive electro-mechanical transducer is generally formed by providing a sacrificial layer whose thickness is equal to the desired distance between the electrodes, forming a vibrating diaphragm above the sacrificial layer, and removing the sacrificial layer.
When medical diagnosis is performed by using a transducer that includes a plurality of elements each of which includes a plurality of electrically connected cells, variations in the sensitivity of the plurality of elements decrease the precision of the diagnosis. Therefore, it is necessary to correct the sensitivity of the individual elements. However, if the sensitivity correction is performed by adjusting the gain in a subsequent circuit as described in Japanese Patent Laid-Open No. 2004-125514, it is necessary to make the dynamic range of the circuit wide. Furthermore, if the variations are too large, correction is impossible.